Adding Users/Contributors

I would like to invite 60 people to become contributors. I added the first two - it was confirmed with a sentence like so and so has recieved an invite.
After attempting to invite the next few people I just get a screen saying please complete the form, with a fresh form. At the bottom I see, the 2 people that have already been invited.
I have tried 5 people and need to invite another 58!